Scientific Imaging Applications

Combustion

Combustion researchers rely on laser-based optical diagnostic techniques as essential tools in understanding and improving the combustion process. For example, a majority of researchers use the quantitative data from planar laser-induced fluorescence (PLIF) techniques in order to study various processes, such as internal combustion engines and hypervelocity combustion.

Laser-Induced Fluorescence in Combustion

In Laser-Induced Fluorescence (LIF), an atom or molecule gets excited by the absorption of a laser photon. When the atom falls back into the ground state after a certain time (typically 1-100nsec), it undergoes radiative emission, known as fluorescence. Since the fluorescence intensity is dependent on parameters such as ground state population, chemical environment, pressure, and temperature, laser-induced fluorescence measurements have become a vital tool in physical chemistry. For example, some applications include the investigation of elementary chemical reactions and trace analytics down to sub-ppm concentrations. In combustion diagnostics, LIF measurements (also called PLIF in planar illumination) are widely utilized. They allow for the qualitative and quantitative detection of flame radicals and combustion intermediates such as OH, C₂, CH, CH₂O, as well as pollutants like NO and CO.

PLIF Instrumentation

In a simple PLIF setup, a thin sheet of laser (typically a pulsed laser) illuminates a flame or flow field. The resulting fluorescence from the radicals is detected by a gated intensified CCD while the excitation is eliminated using a band pass filter. The spatial, temporal, and intensity information of the fluorescence is useful in engine diagnostics techniques for the measurement of local fuel concentrations and temperature distributions. Although naturally occurring chemical species are often used in PLIF measurements, external tracer particles are also occasionally added to create a fluorescence signal.